Build the FIRSTTOUCH overflow trigger flagged in FABRIC-2.md §I.2

The overflow-triggered migration path (a WIREBIND-attached identity's own
drive running low on space) was scoped but never built -- only the
trigger-detection call site was missing, per this section's own text.

- capsule_wirebind.c now tracks the attached blkio_dev* alongside the
  already-tracked VM id, set in try_attach() and cleared in both
  EJECT/UNCLEAN paths.
- New capsule_wirebind_overflow_idle_check(), called once per idle tick
  in repl.c right alongside blk_migration_idle_check() (same cadence):
  reads the attached drive's free/total via blk_get_device_free_blocks(),
  and if free space is below a fixed 10% threshold, extends the
  identity's pool with a one-time blk_firsttouch_claim() of 8 additional
  devblocks on Artemis's system-resident device.
- New blk_owner_has_claim(owner_fp) in block_subsystem.c answers the
  debounce question blk_firsttouch_claim()'s own doc comment had left
  open: a disk scan, not a RAM flag, so the already-extended answer
  survives reboot/reattach, matching BMAPFMT's "ownership travels with
  the block" model.

Premise checked before building (does a WIREBIND-attached drive actually
give a real free/total signal, or does it stay PROVISIONAL/raw): traced
repl.c's attach sequence and confirmed blk_subsys_attach_device() runs on
the same dev pointer right after WIREBIND, and a WIREBIND-eligible drive
is always already STFR/v2-formatted, so the signal is real. Premise held,
unlike the BAM item's overstated one.

Verified with the mandatory 3-arch QEMU acceptance (identical dictionary
hashes, no regression) plus a live logic test of blk_owner_has_claim():
a temporary TEST-OWNER-CLAIM word, run once via SK_CMD and reverted,
confirmed it correctly detects the claiming owner and rejects an
unrelated one. The low-disk-space-triggers-a-claim path itself is not
verified end-to-end -- that needs a real minted WIREBIND-user thumbdrive
with deliberately tiny capacity, out of scope for this pass; noted as
such in the FABRIC-2.md §I.2 closure note rather than overclaimed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018EjXFo7mPXjUMjfJeuUUz4

src/block_subsystem.c

@@ -1481,6 +1481,31 @@ int blk_firsttouch_claim(const uint8_t owner_fp[8], uint32_t count, uint32_t *ou
     return BLK_OK;
 }
 
+/* blk_owner_has_claim - FABRIC-2.md §I.2, 2026-09-05. Disk-scan answer to
+ * "does owner_fp already own a claim," per the decision recorded in
+ * block_subsystem.h: matching BLK_FLAG_CLAIMED devblock, not a RAM flag. */
+int blk_owner_has_claim(const uint8_t owner_fp[8]) {
+    if (!owner_fp) return 0;
+
+    blk_dev_slot_t *slot = first_disk_slot();
+    if (!slot) return 0;
+
+    for (uint32_t lbn = slot->start_lbn;
+         lbn < slot->start_lbn + slot->user_blocks;
+         lbn++) {
+        uint32_t rel_pbn = lbn_to_slot_pbn(slot, lbn);
+        if (slot_pbn_pack_offset(rel_pbn) != 0) continue;
+
+        blk_meta_t meta;
+        if (blk_get_meta(lbn, &meta) != BLK_OK) continue;
+        if ((meta.flags & BLK_FLAG_CLAIMED) &&
+            memcmp(meta.owner_fp, owner_fp, 8) == 0) {
+            return 1;
+        }
+    }
+    return 0;
+}
+
 int blk_meta_relocate_devblock(uint32_t home_devblock, uint32_t target_devblock) {
     if (home_devblock == target_devblock) return BLK_EINVAL;